1. Field of the Invention
The present invention generally relates to a method of and a system for controlling uniformity when etching semiconductor wafers, and more particularly to a method of and a system for controlling a temperature of an electrode (e.g., an electrode of any of: silicon, anodized aluminum, carbon and silicon carbide).
2. Discussion of the Background
Plasma systems for the etching of silicon process wafers are in widespread usage. In some embodiments, such plasma systems have an electrode plate attached to the upper electrode surface for two primary purposes: (1) to present a surface of a material compatible with the process specific chemistry; and (2) to act as a source for scavenging species in the plasma For example, in some applications where an oxide film is etched from a surface of a semiconductor wafer, RF power is applied to the plasma from an upper aluminum electrode, through a silicon electrode plate. The silicon electrode plate enhances the selectivity of oxide-to-silicon etching because when the silicon electrode plate is bombarded by high energy ions from the plasma, silicon is introduced into the system. The silicon subsequently scavenges fluorine radicals and in doing so, the selectivity of the etching of oxide to the etching of silicon can be improved.
Known plasma etchers using silicon electrodes to achieve good selectivity of oxide-to-silicon etching have the silicon electrodes heated by the energy imparted thereto from the RF plasma However, a drawback of such etchers is that the selectivity of the oxide-to-silicon etch is very sensitive to the temperature of the silicon plate. Since the silicon plate in some reactors is heated only by the energy imparted thereto by the plasma, the heat-up time for the silicon plate to reach steady-state conditions is generally long, especially when compared to the relatively short etch time. Therefore, it takes several semiconductor wafers to be etched before the silicon electrode reaches the appropriate temperature for steady-state conditions. This results in the undesirable so-called “first wafers effect,” wherein the first wafers to be etched are unusable thereafter because the silicon electrode has not been heated to a temperature where it is fully effective.
Historically, a solution to the “first wafers effect” has been to run “dummy” wafers through the system to bring the electrode up to the desired operating temperature before committing device wafers to the process. However, this solution is wasteful of silicon material, time, throughput of the machine, and money.
Another solution has been to use heaters to heat the process wafer in the plasma chamber. For example, the stripping of photoresist by hydrogen plasma, which heats the process wafer to a temperature of between 100° C. and 225° C., is disclosed in U.S. Pat. No. 4,201,579. On the other hand, U.S. Pat. No. 5,215,619 discloses the heating of the anode surfaces (i.e., both the walls and the gas manifold) to achieve a high etching rate and in-situ self-cleaning capability. U.S. Pat. No. 5,487,786 (hereinafter “the '786 patent”) discloses heating of a high frequency application electrode to reduce the formation of silicon powder contamination during the chemical vapor deposition of silicon-hydrogen (Si—H). However, the electrode heaters of the '786 patent are in two parts, wherein a first part is an annular ring clamped to the edge of the front side of the electrode and the second part is physically separated from, but in close proximity to, the sides and back of the electrode.
Another drawback of conventional plasma etch apparatus and methods is that there is often poor thermal contact between the electrode and the upper electrode structure. Because of the poor thermal contact between the electrode and the upper electrode structure, the temperature of the electrode may not be uniform. Typically, the electrode exhibits strong temperature non-uniformities, particularly a strong radial temperature gradient, which will cause an undesirable radial etching non-uniformity resulting in non-uniform etching on the process wafers.
Thus, both the “first wafers effect” and the “non-uniform etching effect” cause significant undesirable problems for wafer-to-wafer uniformity and spatial uniformity of the etch rate and etch selectivity.